www.innerbody.comThis site separates out each of the systems in the human body (skeletal, digestive, muscular, etc) and gives some great illustrations and realistic animations.
All in all a good site - with one drawback - there's quite a bit of advertising, and some of it is mixed in pretty tightly with the content - so just be sure you're clicking on the content links and not ads. If you can get by this problem, it's really a very informative site.